Fec frame structuring method and fec multiplexer

ABSTRACT

In an FEC frame structuring method, and an FEC multiplexer, the order of information is changed by a first interleaving circuit  32,  a first error correction code is generated by an RS (239, 223) coding circuit  33,  the order is rechanged to an original order by a first deinterleaving circuit  34,  and a second error correction code is generated by an RS (255, 239) coding means  5.  The second error correction code is decoded by an RS (255, 239) decoding circuit  11  to correct an error of information, the order of information is changed by a second interleaving circuit  35 , the first error correction code is decoded by an RS (239, 223) decoding circuit  36  to correct a residual error of information, and the order is rechanged to an original order by a second interleaving circuit  37.

[0001] This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/JP00/08635 which has an International filing date of Dec. 6, 2000, which designated the United States of America and was not published in English.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] This invention relates to an FEC structuring method and an FEC multiplexer, designed to realize long-haul and large-capacity transmission by correcting a bit error caused by the degradation of optical SNR based on forward error correction (FEC) in an optical transmission system.

[0004] 2. Background Art

[0005]FIG. 1 is a schematic diagram showing a conventional FEC multiplexer, for example one presented in accordance with ITU-T Recommendation G.975. In the drawing, a reference numeral 1 denotes a first demultiplexing circuit for demultiplexing 2.5 Gbit/s STM-16 data into 16 parallel 156 Mbit/s data; and 2 a second demultiplexer for demultiplexing the 16 parallel 156 Mbit/s data into 128 parallel 19 Mbit/s data. A reference numeral 3 denotes a first speed conversion circuit for adding a redundant information region to the 128 parallel 19 Mbit/s data, and increasing a transmission speed to 21 Mbit/s by an amount equal to the addition of the redundant information region; and 4 an overhead insertion circuit for inserting overhead information, e.g., frame alignment information, necessary for the operation and maintenance of the optical transmission system, into the redundant information region. A reference numeral 5 denotes an RS (255, 239) coding circuit, represented by Reed-Solomon (RS) code, for generating an error correction code for the overhead information and the STM-16 data, and storing redundant information thereof in the redundant information region; 6 a first multiplexer for multiplexing the STM-16 data having the overhead information and the error correction code added thereto into 16 parallel 167 Mbit/s data; and 7 a second multiplexing circuit for multiplexing the 16 parallel 167 Mbit/s into 2.66 Gbit/s FEC frame.

[0006] A reference numeral 8 denotes a third demultiplexing circuit for demultiplexing the 2.66 Gbit/s FEC frame, which has been multiplexed by the second multiplexing circuit 7 and transmitted through an optical transmission path, into 16 parallel 167 Mbit/s data: and 9 a fourth demultiplexing circuit for demultiplexing the 16 parallel 167 Mbit/s data into 128 parallel 21 Mbit/s data. A reference numeral 10 denotes a frame alignment circuit for detecting the head position of the 128 parallel 21 Mbit/s data according to the overhead information of the redundant information region added to the 128 parallel 21 Mbit/s data; and 11 an RS (255, 239) decoding circuit for decoding the 128 parallel 21 Mbit/s data, detecting a data error in the FEC frame according to the decoding of the error correction code of the added redundant information region, and correcting the data into original correct data. A reference numeral 12 denotes an overhead separation circuit for separating the overhead information from the redundant information region of the 128 parallel 21 Mbit/s data; 13 a second speed conversion circuit for eliminating the redundant information region from the 128 parallel 21 Mbit/s data, and reducing a transmission speed to 19 Mbit/s by an amount equal to the elimination of the redundant information region; 14 a third multiplexing circuit for multiplexing the 128 parallel 19 Mbit/s data to 16 parallel 156 Mbit/s data; and 15 a fourth multiplexing circuit for multiplexing the 16 parallel 156 Mbit/s data into 2.5 Gbit/s STM-16 data.

[0007] Next, an operation will be described.

[0008] In FIG. 1, the first demultiplexing circuit 1 demultiplexes 2.5 Gbit/s ASTM-16 data into 16 parallel 156 Mbit/s. The second demultiplexing circuit 2 demultiplexes the 16 parallel 156 Mbit/s data into 128 parallel 19 Mbit/s data. The first speed conversion circuit 3 adds a redundant information region to the 128 parallel 19 Mbit/s data, increases a transmission speed by an amount equal to the addition of the redundant information region, and converts the data into 128 parallel 21 Mbit/s data. The overhead insertion circuit 4 inserts overhead information, e.g., frame alignment information or the like, necessary for the operation and maintenance of the optical transmission system, into the redundant information region. The RS (255, 239) coding circuit 5 generates an error correction code for the overhead information and the STM-16 data, and stores the redundant information thereof in the redundant information region. The first multiplexing circuit 6 multiplexes the overhead information and the STM-16 data having the redundant information added thereto into 16 parallel 167 Mbit/s data. Further, the second multiplexing circuit 7 multiplexes the 16 parallel 167 Mbit/s data into a 2.66 Gbit/s FEC frame, and transmits the data to the optical transmission path.

[0009] The third demultiplexing circuit 8 demultiplexes the 2.66 Gbit/s FEC frame obtained by the multiplexing carried out by the second multiplexing circuit 7 and transmitted through the optical transmission path, into 16 parallel 167 Mbit/s data. Then, the fourth demultiplexing circuit 9 demultiplexes the 16 parallel 167 Mbit/s data into 128 parallel 21 Mbit/s data. The frame alignment circuit 10 detects the head position of the 128 parallel 21 Mbit/s data according to the overhead information of the redundant information region added to the 128 parallel 21 Mbit/s data. The RS (255, 239) decoding circuit 11 decodes the 128 parallel 21 Mbit/s data, detects a data error in the FEC frame according to the decoding of the redundant information of the error correction circuit of the added redundant information region, and corrects the data into original correct data. The overhead separation circuit 12 separates the overhead information from the redundant information region of the 128 parallel 21 Mbit/s data. The second speed conversion circuit 13 erases the redundant information region from the 128 parallel 21 Mbit/s data, reduces a transmission speed to 19 Mbit/s by an amount equal to the erasure of the redundant information region. The third multiplexing circuit 14 multiplexes the 128 parallel 19 Mbit/s into 16 parallel 156 Mbit/s data. Then, the fourth multiplexing circuit 15 multiplexes the 16 parallel 156 Mbit/s data into 2.5 Gbit/s STM-16 data.

[0010]FIG. 2 is a structural view showing an FEC frame outputted from the overhead insertion circuit. FIG. 3 is a structural view showing FEC frames outputted from the RS (255, 239) coding circuit and the second multiplexing circuit.

[0011] As shown in FIG. 2, the FEC frame outputted from the overhead insertion circuit 4 is structured of subframes 1 to 128 including 1-bit overhead information, 238-bit STM-16 data, and a 16-bit RS (255, 239) error correction code. As shown in FIG. 3, by the RS (255, 239) coding circuit 5, error correction coding is executed for every 8 subframes. For example, in the subframes 1 to 8, error correction codes ERO-0 to ERO-15 are computed for the overhead information and the STM-16 data, and stored in the 16-bit RS (255, 239) redundant information region. In addition, the FEC frame outputted from the second multiplexing circuit 7 is generated by sequentially multiplexing the subframes 1 to 128.

[0012] Here, f is a given natural number, indicating the number of times of multiplexing for each of the error correction codes ERO-0 to ERO-15. In each of FIGS. 2 and 3, the example of f=16 is shown.

[0013] In the described FEC frame, the FEC frame outputted from the second demultiplexing circuit 2 is 238-bit STM-16 data. By the first speed conversion circuit 3, the redundant information region composed of 1-bit overhead information and 16-bit RS (255, 239) redundant information is added, and a transmission speed is increased by an amount equal to the addition of the redundant information region. Accordingly, the transmission speed is increased by 255/238 times more than that of the original STM-16 data, and the transmission speed of the FEC frame is changed from 2.5 Gbit/s to 2.6 Gbit/s.

[0014] The FEC frame having the foregoing structure enables a bit error to be corrected. Thus, in the optical transmission system having degraded optical SNR, a high-quality service can be provided. It is therefore possible to build a long-haul or large-capacity optical transmission system.

[0015] In the FEC frame structure shown in each of FIGS. 2 and 3, by shortening the STm-16 data in the subframe from 238 bits to 110 bits, and setting RS (255, 239) error correction coding to be RS (127, 111) error correction coding, a ratio of error correction coding to target information is increased. Thus, it is possible to enhance an error correction capability.

[0016] Since the conventional FEC frame structuring method and FEC multiplexer are constructed in the foregoing manner, if the transmission distance of the optical transmission path is long, or if the number of wavelengths is increased in a wavelength multiplexing system, optical SNR is greatly degraded. To compensate for this degradation, correction can be made to a certain extent, for example by increasing a ratio of an error correction code to target information. However, the increased ratio of the error correction code to the target information necessitates a further increase in the raised ratio of the transmission speed by the first speed conversion circuit 3. For example, in the RS (127, 111) error correction coding, the transmission speed of the FEC frame is 2.89 Gbit/s, larger by 127/110 times than that of STM-16 data set to 2.5 Gbit/s, increasing the degradation amount of an optical transmission characteristic. Thus, even when the radio of the error correction code to the target information was increased, it was impossible to build a long-haul and large-capacity optical transmission system having a predetermined quality.

[0017] The present invention was made to solve the foregoing problems. Objects of the invention are to provide an FEC frame structuring method and an FEC multiplexer, capable of increasing a transmission speed even when a ratio of an error correction code to information is increased, and greatly enhancing an error correction capability even when the degradation amount of an optical transmission characteristic is increased.

DISCLOSURE OF THE INVENTION

[0018] In accordance with the present invention, there is provided an FEC frame structuring method, comprising: a first error correction code generation step of generating a first error correction code by coding information in subframes amounting to n in number for every m subframes, and adding the generated first error correction code; an interleaving step of dividing the subframes amounting to n in number, each having the added first error correction code for every m subframes, and interleaving information for a subframe of n/m unit by l times; a second error correction code generation step of generating a second error correction code by coding information in the subframes amounting to n in number for every m subframes, and adding the generated second error correction code; and an FEC frame generation step of generating an FEC fame by multiplexing the subframes amounting to n in number, each having the added second error correction code by n times.

[0019] Since two kinds of error correction codes, first and second, having bits of information to be subjected to error correction coding switched, are added, at a receiving side, by decoding these two kinds of first and second error correction codes, a residual error unable to be corrected by one kind of error correction code can be corrected. Thus, it is possible to greatly enhance an error correction capability even when a ratio of an error correction code to information is increased, consequently increasing a transmission speed and the degradation amount of an optical transmission characteristic.

[0020] In accordance with the invention, there is provided an FEC frame structuring method, comprising: an interleaving step of dividing subframes amounting to n in number for every m subframes, and interleaving information in a subframe of n/m unit by a number of times 1 to generate a subframe; a first error correction code generation step of generating a first error correction code by coding the information in each of the subframes amounting to n in number for every m subframes, and adding the generated first error correction code; a deinterleaving step of deinterleaving the subframes amounting to n in number, each having the added first error correction code by l times, and returning the information interleaved in the interleaving step to original information; a second error correction code generation step of generating a second error correction code by coding the information in each of the subframes amounting to n in number for every m subframes, and adding the generated second error correction code; and an FEC frame generation step of generating an FEC frame by multiplexing each of the subframes amounting to n in number, each having the added second error correction code by n times.

[0021] Since two kinds of error correction codes, first and second, having bits of information to be subjected to error correction coding switched, are added, at a receiving side, by decoding the two kinds of first and second error correction codes, a residual error unable to be corrected by one kind of error correction code can be corrected. Thus, it is possible to greatly enhance an error correction capability even when a ratio of an error correction code to information is increased, consequently increasing a transmission speed and the degradation amount of an optical transmission characteristic.

[0022] In addition, since an FEC frame is structured by deinterleaving, in the deinterleaving steps, the information interleaved in the interleaving step to return it to original information. Thus, it is possible to transmit the FEC frame of the original information, the order thereof having not been changed.

[0023] According to the FEC frame structuring method of the invention, l is n/m.

[0024] Thus, since the number of times of interleaving is set equal to the unit number of subframes, bits of information to be subjected to error correction coding are uniformly changed, and errors of information are uniformly distributed. Thus, the error correction capability can be further enhanced, and the configuration of a device for generating an FEC frame by this FEC frame structuring method can be simplified and miniaturized.

[0025] According to the FEC frame structuring method of the invention, l is larger than n/m by k times, and with the number k of FEC frames set as one cycle, interleaving is executed by n/m times for different information of each FEC frame.

[0026] Thus, it is possible to further enhance the error correction capability while an increase rate of a transmission speed is maintained constant.

[0027] According to the FEC frame structuring method of the invention, the first and second error correction codes are Reed-Solomon codes respectively represented by RS (q, r) and RS (p, q) (p, q and r are natural numbers, and p>q>r, p representing a code length of the second error correction code, q an information length of the second error correction code, and a code length of the first error correction code, and r an information length of the first error correction code).

[0028] Thus, it is possible t o facilitate the structuring of an FEC frame capable of enhancing the error correction capability while the increase rate of the transmission speed is maintained constant according to the above-described condition.

[0029] In accordance with the invention, there is provided an FEC multiplexer, comprising: first error correction coding means for generating a first error correction code for parallel information, the order thereof having been changed by first interleaving means, and then storing the generated first error correction code in a redundant information region; second error correction coding means for generating a second error correction code for the parallel information order-rechanged by first deinterleaving means, and storing the generated second error correction code in the redundant information region; second error correction decoding means for correcting an error of the parallel information by decoding the second error correction code stored in the redundant information region of the parallel information of an FEC frame transmitted through a transmission path; first error correction decoding means for correcting a residual error of the parallel information by decoding the first error correction code stored in the redundant information region of the parallel information order-changed by second interleaving means; and second deinterleaving means for rechanging the order, changed by the second interleaving means, of the parallel information to an original order.

[0030] Thus, since the two kinds of error correction codes, first and second, bits of information thereof to be subjected to error correction coding switched, are added to the FEC frame transmitted through the optical transmission path, at a receiving side, by decoding the two kinds of first and second error correction codes, a residual error unable to be corrected by one kind of error correction code can be corrected. As a result, even when a ratio of an error correction code to information is increased, consequently increasing the transmission speed and the degradation amount of an optical transmission characteristic, it is possible to greatly enhance an error correction capability.

[0031] Moreover, since the information interleaved by the first interleaving means is deinterleaved by the first deinterleaving means, it is possible to transmit the FEC frame of original information, the order thereof having not been changed, to the optical transmission path.

[0032] According to the FEC multiplexer of the invention, error correction means composed of the second error correction decoding means, the second interleaving means, the first error correction decoding means, and the second deinterleaving means is connected in a multistage and cascaded manner.

[0033] Thus, only by adding the error correction means, without changing the structure of the FEC frame, it is possible to further enhance the error correction capability by repeating error correction for the two kinds of error correction codes by a number of times.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a schematic diagram showing a conventional FEC multiplexer.

[0035]FIG. 2 is a structural view showing an FEC frame outputted from an overhead insertion circuit.

[0036]FIG. 3 is a structural view showing FEC frames outputted respectively from an RS (255, 239) coding circuit and a second multiplexing circuit.

[0037]FIG. 4 is a schematic diagram showing an optical transmission system, to which an FEC frame structuring method and an FEC multiplexer according to a first embodiment of the invention are applied.

[0038]FIG. 5 is a schematic diagram showing an FEC multiplexer of the first embodiment of the invention.

[0039]FIG. 6 is a structural view showing an FEC frame outputted from an overhead insertion circuit.

[0040]FIG. 7 is a structural view showing an FEC frame outputted from an RS (239, 223) coding circuit.

[0041]FIG. 8 is a structural view showing FEC frames outputted respectively from the RS (255, 239) coding circuit and a second multiplexing circuit.

[0042]FIG. 9 is a schematic diagram showing an FEC frame multiplexer according to a second embodiment of the invention.

[0043]FIG. 10 is a view illustrating an FEC frame structuring method according to a third embodiment of the invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0044] Next, detailed description will be made of the preferred embodiments of the present invention with reference to the accompanying drawings.

[0045] (First Embodiment)

[0046]FIG. 4 is a schematic diagram showing an optical transmission system, to which an FEC frame structuring method and an FEC multiplexer according to the first embodiment of the invention are applied. In the drawing, a reference numeral 21 denotes a first optical receiver for receiving an STM-16 optical signal, and converting the optical signal into an electric signal; 22 an FEC multiplexing circuit (FEC multiplexer) for demultiplexing the electric signal from the first optical receiver 21, executing the insertion of overhead information, error correction coding, and so on, and then executing multiplexing again to structure an FEC frame; and 23 a first optical transmitter for converting the FEC frame into an optical signal. A reference numeral 24 denotes an optical transmission path for transmitting the FEC frame of the optical signal; 25 a second optical receiver for converting the FEC frame transmitted through the optical transmission path 24 from the optical signal into an electric signal; 26 an FEC demultiplexing circuit (FEC multiplexer) for demultiplexing the electric signal from the optical receiver 25, executing frame alignment of the FEC frame, decoding of an error correction code, separation of overhead information and so on, and then executing multiplexing again; and 27 a second optical transmitter for converting the electric signal from the FEC demultiplexing circuit 26 into an optical signal, and outputting the STM-16 optical signal.

[0047]FIG. 5 is a schematic diagram showing the FEC multiplexer of the first embodiment of the invention. In the drawing, the upper side shows the FEC multiplexing circuit 22 of FIG. 4, and the lower side shows the FEC demultiplexing circuit 26 of FIG. 4. In the drawing, a reference numeral 1 denotes a first demultiplexing circuit (first demultiplexing means) for demultiplexing 2.5 Gbit/s STM-16 data into 16 parallel 156 Mbit/s data; and 2 a second demultiplexing circuit (first demultiplexing means) for demultiplexing the 16 parallel 156 Mbit/s data into 128 parallel 19 Mbit/s data. A reference numeral 31 denotes a first speed conversion circuit (first speed conversion means) for adding a redundant information region to the 128 parallel 19 Mbit/s data, and increasing a transmission speed to 22 Mbit/s by an amount equal to the addition of the redundant information region; and 4 an overhead insertion circuit (overhead insertion means) for inserting overhead information, e.g., frame alignment information, necessary for the operation and maintenance of the optical transmission system, into the redundant information region. A reference numeral 32 denotes a first interleaving circuit (first interleaving means) for changing the order of the 128 parallel 22 Mbit/s data; and 33 an RS (239, 223) coding circuit (first error correction coding means) for executing RS (239, 223) error correction coding, and storing the redundant information of the error correction code in the redundant information region. A reference numeral 34 denotes a first deinterleaving circuit (first deinterleaving means) for rechanging the order of the 128 parallel 22 Mbit/s data, changed by the first interleaving circuit 32, to an original order; and 5 an RS (255, 239) coding circuit (second error correction coding means) for executing RS (255, 239) error correction coding, and storing the redundant information of the error correction code in the redundant information region. A reference numeral 6 denotes a first multiplexing circuit (first multiplexing means) for multiplexing the 128 parallel 22 Mbit/s data into 16 parallel 179 Mbit/s data; and 7 a second multiplexing circuit (first multiplexing means) for multiplexing the 16 parallel 179 Mbit/s data into 2.86 Gbit/s FEC frame.

[0048] A reference numeral 8 denotes a third demultiplexing circuit (second demultiplexing means) for demultiplexing the 2.86 Gbit/s FEC frame into 16 parallel 179 Mbit/s data; 9 a fourth demultiplexing circuit (second demultiplexing means) for demultiplexing the 16 parallel 179 Mbit/s data into 128 parallel 22 Mbit/s data; and 10 a frame alignment circuit (frame alignment means) for detecting the head position of the 128 parallel 22 Mbit/s data according to the overhead information stored in the redundant information region. A reference numeral 40 denotes an error correction circuit (error correction means), which includes: an RS (255, 239) decoding circuit (second error correction decoding means) 11 for decoding the 128 parallel 22 Mbit/s data, and correcting a bit error according to the decoding of the RS (255, 239) error correction code; a second interleaving circuit (second interleaving means) 35 for changing the order of the 128 parallel 22 Mbit/s data similarly to that of the first interleaving circuit 32; an RS (239, 223) decoding circuit (first error correction decoding means) 36 for decoding the 128 parallel 22 Mbit/s data excluding the RS (255, 239) redundant information, and correcting a bit error according to the decoding of the RS (239, 223) error correction code; and a second deinterleaving circuit (second deinterleaving means) 37 for rechanging the order of the 128 parallel 22 Mbit/s data, changed by the second interleaving circuit 35, to an original order. A reference numeral 12 denotes an overhead separation circuit (overhead separation means) for separating the overhead information from the redundant information region; and 38 a second speed conversion circuit (second speed conversion means) for erasing the redundant information region from the 128 parallel 22 Mbit/s data, and reducing the transmission speed to 19 Mbit/s by an amount equal to the erasure of the redundant information region. A reference numeral 14 denotes a third multiplexing circuit (second multiplexing means) for multiplexing the 128 parallel 19 Mbit/s data into 16 parallel 156 Mbit/s data; and 15 a fourth multiplexing means (second multiplexing means) for multiplexing the 16 parallel 156 Mbit/s data into 2.5 Gbit/s STM-16 data.

[0049] Next, an operation will be described.

[0050] In FIG. 4, the first optical receiver 21 receives an STM-16 optical signal, and converts the optical signal into an electric signal. The FEC multiplexing circuit 22 demultiplexes the electric signal obtained by the conversion, stores overhead information and an error correction code in the redundant information region, and executing multiplexing again to structure an FEC frame. The first optical transmitter 23 converts the FEC frame into an optical signal, and sends the optical signal to the optical transmission path 24 made of an optical fiber. The second optical receiver 25 converts the optical signal of the FEC frame transmitted through the optical transmission path 24 into an electric signal. The FEC demultiplexing circuit 26 demultiplexes the electric signal obtained by the conversion, frame-synchronizes the FEC frame according to the overhead information stored in the redundant information region, decodes the error correction code to correct a bit error, and then executes multiplexing again after the redundant information region is erased. The second optical transmitter 27 converts the electric signal from the FEC demultiplexing circuit 26 into an optical signal, and outputs the STM-16 optical signal. In this case, since optical SNR is degraded because of the long-haul and large-capacity transmission of the optical signal on the optical transmission path 24, the FEC frame outputted from the second optical receiver 25 has a number of bit errors that have occurred. Such a bit error is corrected by the FEC demultiplexing circuit 26, and a bit error rate of the STM-16 optical signal outputted from the second optical transmitter 27 is greatly improved. Thus, communication services having predetermined quality can be provided.

[0051] In the upper side of FIG. 5, i.e., in the FEC multiplexing circuit 22, the first demultiplexing circuit 1 demultiplexes 2.5 Gbit/s STM-16 data into 16 parallel 156 Mbit/s data, and the second demultiplexing circuit 2 demultiplexes the 16 parallel 156 Mbit/s data into 128 parallel 19 Mbit/s data. The first speed conversion circuit 31 adds a redundant information region to the 128 parallel 19 Mbit/s data, and increases a transmission speed to 22 Mbit/s by an amount equivalent to the addition of the redundant information region. The overhead insertion circuit 4 inserts overhead information, e.g., frame alignment information, necessary for the operation and maintenance of the optical transmission system, into the redundant information region. The first interleaving circuit 32 changes the order of the 120 parallel 22 Mbit/s data, and the RS (239, 223) coding circuit 33 executes RS (239, 223) error correction coding, and stores the redundant information of the error correction code in the redundant information region. The first deinterleaving circuit 34 rechanges the order of the 128 parallel 22 Mbit/s data, changed by the first interleaving circuit 32, to an original order. The RS (255, 239) coding circuit 5 executes RS (255, 239) error correction coding, and stores the redundant information of the error correction code in the redundant information region. The first multiplexing circuit 6 multiplexes the 128 parallel 22 Mbit/s data into 16 parallel 179 Mbit/s data, and the second multiplexing circuit 7 multiplexes the 16 parallel 179 Mbit/s data, and outputs a 2.86 Gbit/s FEC frame.

[0052] On the other hand, in the lower side of FIG. 5, i.e., in the FEC demultiplexing circuit 26, the third demultiplexing circuit 8 demultiplexes the 2.8 Gbit/s FEC frame into 16 parallel 179 Mbit/s data, and the fourth demultiplexing circuit 9 demultiplexes the 16 parallel 179 Mbit/s data into 128 parallel 22 Mbit/s data. The frame alignment circuit 10 evaluates the overhead information stored in the redundant information region, and detects the head position of the 128 parallel 22 Mbit/s data. The RS (255, 239) decoding circuit 11 in the error correction circuit 40 decodes the 128 parallel 22 Mbit/s data, and correct a bit error according to the decoding of the RS (255, 239) error correction code. The second interleaving circuit 35 changes the order of the 128 parallel 22 Mbit/s data similarly to that of the first interleaving circuit 32. The RS (239, 223) decoding circuit 36 decodes the 128 parallel 22 Mbit/s data excluding the RS (255, 239) redundant information, and corrects a bit error according to the decoding of the RS (239, 223) error correction code. The second deinterleaving circuit 37 rechanges the order of the 128 parallel 22 Mbit/s data, changed by the second interleaving circuit 35, to an original order. The overhead separation circuit 12 separates the overhead information from the redundant information region, and the second speed conversion circuit 38 erases the redundant information region from the 128 parallel 22 Mbit/s data, and reduces the transmission speed to 19 Mbit/s by an amount equal to the erasure of the redundant information region. The third multiplexing circuit 14 multiplexes the 128 parallel 19 Mbit/s data into 16 parallel 156 Mbit/s data. The fourth multiplexing circuit 15 multiplexes the 16 parallel 156 Mbit/s data, and outputs 2.5 Gbit/s STM-16 data.

[0053]FIG. 6 is a structural view showing an FEC frame outputted from the overhead insertion circuit; FIG. 7 a structural view showing an FEC frame outputted from the RS (239, 223) coding circuit; and FIG. 8 a structural view showing an FEC frame outputted from the RS (255, 239) coding circuit.

[0054] As shown in FIG. 6, the 128 parallel 22 Mbit/s data outputted from the overhead insertion circuit 4 is composed of subframes amounting to n in number (n=128) including 1-bit overhead information, 222-bit STM-16 data, 16-bit RS (239, 223) redundant information, and 16-bit RS (255, 239) redundant information. In this case, it is assumed that the STM-16 data is set as transmission information, and the overhead information, the RS (239, 223) redundant information, and the RS (255, 239) redundant information are stored in the redundant information region.

[0055] At the overhead insertion circuit 4, overhead information is stored in the overhead information region of each of the subframes 1 to 128. In FIG. 6, the subframes 1 to 128 are divided for every m (m=8) subframes, and bits of overhead information OH 1 to OH 16 are stored in the overhead information regions of the subframes of n/m units (128/8=16).

[0056] At the first interleaving circuit 32, the 128 parallel 22 Mbit/s data shown in FIG. 6 is divided for every m (m=8) subframes as in the case of the 128 parallel 22 Mbit/s data shown in FIG. 7, and the order of the data of the subframes of n/m units (128/8=16). Here, as an example of data order changing, the case of performing interleaving by l times (l=16) is shown, specifically, no changing in a first row, shifted up by one for every 8 subframes in a second row, shifted up by two in a third row, . . . , shifted up by fifteen in a sixteenth row, and shifted up by sixteen in a seventeenth row (equal to no changing) (interleaving step).

[0057] The RS (239, 223) coding circuit 33 generates an RS (239, 223) error correction code for each of 8 subframes targeting the overhead information and the STM-16 data as in the case of the 128 parallel 22 Mbit/s data shown in FIG. 7, and stores the redundant information of the error correction code in the RS (239, 223) error correction code area of the redundant information region (first error correction code generation step).

[0058] At the first deinterleaving circuit 34, for the 128 parallel 22 Mbit/s data shown in FIG. 7 as in the case of the 18 parallel 22 Mbit/s data shown in FIG. 8, the order of data is changed for every 8 subframes in such a way as to set the data region of the STM-16 to an original order (interleaving step, and deinterleaving step).

[0059] The RS (255, 239) coding circuit 5 generates an RS (255, 239) error correction code for each of 8 subframes targeting the overhead information, the STM-16 data and the RS (239, 223) redundant information as in the case of the 128 parallel 22 Mbit/s data shown in the upper side of FIG. 8, and stores the redundant information of the error correction code in the redundant area of the RS (255, 239) error correction code of the redundant information region (second error correction code generation step).

[0060] The 128 parallel 22 Mbit/s data shown in the upper side of FIG. 8 is subjected to multiplexing by the first and second multiplexing circuits 6 and 7, and a 2.86 Gbit/s FEC frame shown in the lower side of FIG. 8 is structured. Here, f represents (n/m=128/8=16).

[0061] In the FEC frame of the first embodiment, with respect to the 2.5 Gbit/s transmission speed of the original STM-16 data, a 33-bit redundant information region is added to the 222-bit STM-16 data, and the transmission speed is increased by 255/222 times. Thus, the transmission speed of the FEC frame is set to 2.86 Gbit/s.

[0062] On the other hand, in the FEC demultiplexing circuit 26, the RS (255, 239) decoding circuit 11 executes RS (255, 239) error correction decoding for each of 8 subframes for the overhead information, the STM-16 data, the RS (239, 223) redundant information, and RS (255, 239) redundant information of the 128 parallel 22 Mbit/s data shown in the upper stage of FIG. 8, and then corrects a bit error in each of the overhead information, the STM-16 data, the RS (239, 223) redundant information, and the RS (255, 239) redundant information according to the decoding of the RS (255, 239) error correction code. In this case, if a number of bit errors exceeding the correction capability has been caused, then a bit error remains in the output data of the RS (255, 239) decoding circuit 11.

[0063] The second interleaving circuit 35 changes the order of the 128 parallel 22 Mbit/s data similarly to that of the first interleaving circuit 32. The RS (239, 223) decoding circuit 36 executes RS (239, 223) error correction decoding for each of 8 subframes for the overhead information, the STM-16 data and the RS (239, 223) redundant information of the 128 parallel 22 Mbit/s data shown in FIG. 7, and thereby corrects a bit error remaining in each code according to the decoding of the RS (239, 223) error correction code. Thus, for the RS (255, 239) error correction code and the RS (239, 223) error correction code, bits of information to be subjected to error correction coding are switched, causing bit errors to be dispersed between the codes. Therefore, the error correction capability can be greatly enhanced.

[0064] The second deinterleaving circuit 37 rechanges the order of the 128 parallel 22 Mbit/s data, changed by the second interleaving circuit 35, to an original order.

[0065] As described above, according to the first embodiment, in the FEC frame structuring method and the FEC multiplexer, two kinds of error correction coding are carried out, the first interleaving circuit 32, the first deinterleaving circuit 34, the second interleaving circuit 35 and the second deinterleaving circuit 37 are disposed, and the order of information is changed between the two kinds of error correction codes. Thus, compared with the structure provided by the conventional technology, an error correction capability can be greatly enhanced, and a long-haul and large-capacity optical transmission system can be built even when a transmission speed is increased to 2.86 Gbit/s. In addition, the first interleaving circuit 32 is disposed before the RS (239, 223) coding circuit 33 of the first stage, and the first deinterleaving circuit 34 is disposed before the RS (255, 239) coding circuit 5 of the next stage. Thus, the STM-16 data in the FEC frame can be transmitted without changing the order thereof.

[0066] Furthermore, the numbers of times of interleaving and deinterleaving (l=16) are set equal to the unit numbers (n/m—128/8=16) of interleaved and deinterleaved subframes. Thus, bits of information to be subjected to error correction coding are uniformly order-changed, and information errors are uniformly dispersed. As a result, it is possible to further enhance the error correction capability, and to easily configure and miniaturize the FEC multiplexer.

[0067] (Second Embodiment)

[0068]FIG. 9 is a schematic diagram showing an FEC multiplexer according to the second embodiment of the invention. This FEC multiplexer comprises an error correction circuit (error correction means) 40 connected in a multistage and cascaded manner. The error correction circuit 40 includes: an RS (255, 239) decoding circuit 11; a second interleaving circuit 35; an RS (239, 223) decoding circuit 36; and second interleaving circuit 37.

[0069] Next, an operation will be described.

[0070] In FIG. 9, the error correction circuit 40 is connected in the multistage and cascaded manner in the FEC demultiplexing circuit 26. According to the second embodiment, the operations of bit error correction are sequentially repeated for two kinds of error correction codes. Thus, the error correction capability can be further enhanced, and it is possible to build a long-haul and large-capacity optical transmission system without changing the configuration of the FEC frame or changing any hardware other than one for connecting the error correction circuit 40 in a multistage and cascaded manner.

[0071] (Third Embodiment)

[0072]FIG. 10 is a view illustrating an FEC frame structuring method according to a third embodiment of the invention. According to this method, with the number k (k=4) of FEC frames set as one cycle, interleaving is executed by a number of n/m (=128/8=16) times by shifting the information of each FEC frame to be interleaved by 16 bits, and interleaving is carried out by totally 1 times (l is larger by k times than n/m: 16×4=64).

[0073] Next, an operation will be described.

[0074] In the foregoing first embodiment, the number of interleaving times l between the two kinds of codes was l=16. However, by setting the number of interleaving times l to be a natural number multiple, e.g., l=32, 48, 64, . . . , it is possible to further enhance the error correction capability while maintaining constant the increase rate of a transmission speed. FIG. 10 shows a case of four FEC frames set as one cycle. In this case, interleaving is carried out by 16 times by shifting information for interleaving each FEC frame by 16 bits, and thus interleaving is carried out by totally 64 times, i.e., 16×4=64.

[0075] As described above, according to the third embodiment, without changing the basic structure of the FEC frame, and by changing only the first interleaving circuit 32, the first deinterleaving circuit 34, the second interleaving circuit 35, and the second deinterleaving circuit 37 regarding hardware, the error correction capability can be enhanced while the increase data of the transmission speed is maintained constant. As a result, it is possible to build a long-haul and large-capacity optical communication system having a large degradation amount of optical SNR.

[0076] (Fourth Embodiment)

[0077] In the foregoing embodiments, the transmission speed of the FEC frame was 2.86 Gbit/s, and the FEC frame was transmitted through the optical transmission path 24. A plurality of FEC frames may be multiplexed, and the transmission speed of an optical signal passed through the optical transmission path 24 may be set to a×2.86 Gbit/s (a is a given natural number). In addition, the STM-16 data of 2.5 Gbit/s was described as transmission information. However, b pieces of FEC multiplexing and demultiplexing circuits 22 and 26 may be disposed to process the FEC frames of a b type (b is a given natural number), and information may be set as data of b×2.5 Gbit/s (e.g., with b=4, 10 Gbit/s STM-64 data). Moreover, an FEC frame structure similar to that provided by the conventional technology can be applied even for the data of transmission speeds set in accordance with other standards. In this case, speeds are set for the FEC frames corresponding to the data of the transmission speeds set in accordance with the other standards.

[0078] (Fifth Embodiment)

[0079] In the foregoing embodiments, the examples of RS (255, 239) and RS (239, 223) represented by Reed-Solomon (RS) codes were taken as the error correction codes. However, for example, RS (255, 239) and RS (239, 207) may be used. In this case, if a code length and an information length of the former RS code are set as p and q, and an information length of the latter RS code is set as r, then RS (p, q) and RS (q, r) may be set, and a bit length of each subframe in the FEC frame may be set to a value corresponding to the values of p, q and r. In addition, the example of the 1-bit overhead information set for each subframe was described. However, overhead information or 2 or more bits may be set for each subframe.

[0080] Industrial Applicability

[0081] As described above, in the FEC frame structuring method and the FEC multiplexer of the present invention, the error correction capability can be greatly enhanced even when the degradation amount of an optical transmission characteristic is increased in the optical transmission system. Thus, the invention is advantageous for realizing long-haul and large-capacity transmission. 

What is claimed is:
 1. An FEC frame structuring method applied to an optical transmission system, comprising: a first error correction code generation step of generating a first error correction code by coding redundant information composed of overhead information, and transmission information in subframes amounting to n (n is a given natural number) in number for every m (m is a natural number, and a factor of n) subframes, and adding the generated first error correction code as redundant information; an interleaving step of dividing the subframes amounting to n in number, each having the added first error correction code generated in the first error correction code generation step, for every m subframes, and interleaving at least one selected from the redundant information composed of the overhead information and the first error correction code of a subframe of n/m unit, and the transmission information by l times (l is a given natural number); a second error correction code generation step of generating a second error correction code by coding the redundant information composed of the overhead information and the first error correction code, and the transmission information in each of the subframes amounting to n in number, interleaved in the interleaving step, for every m subframes, and adding the generated second error correction code as redundant information; and an FEC frame generation step of generating an FEC fame by multiplexing the subframes amounting to n in number, each having the second error correction code added in the second error correction code generation step.
 2. An FEC frame structuring method applied to an optical transmission system, comprising: an interleaving step of dividing subframes amounting to n (n is a given natural number) in number for every m (m is a natural number, and a factor of n) subframes, and interleaving at least one selected from redundant information composed of overhead information, and transmission information in a subframe of n/m unit by l times (l is a given natural number); a first error correction code generation step of generating a first error correction code by coding the redundant information composed of the overhead information, and the transmission information in each of the subframes amounting to n in number, interleaved in the interleaving step, for every m subframes, and adding the generated first error correction code as redundant information; a deinterleaving step of deinterleaving the subframes amounting to n in number, each having the first error correction code added in the first error correction code generation step, by l times, and returning the information interleaved in the interleaving step to original information; a second error correction code generation step of generating a second error correction code by coding the redundant information composed of the overhead information and the first error correction code, and the transmission information in each of the subframes amounting to n in number, deinterleaved in the deinterleaving step, for every m subframes, and adding the generated second error correction code as redundant information; and an FEC frame generation step of generating an FEC frame by multiplexing each of the subframes amounting to n in number, each having the second error correction code added in the second error correction code generation step, by n times.
 3. An FEC frame structuring method according to claim 1, wherein l is n/m.
 4. An FEC frame structuring method according to claim 2, wherein l is n/m.
 5. An FEC frame structuring method according to claim 1, wherein l is larger than n/m by k times (k is a given natural number), and with the number k of FEC frames set as one cycle, interleaving is executed by n/m times for different information of each FEC frame.
 6. An FEC frame structuring method according to claim 2, wherein l is larger than n/m by k (k is a given natural number) times, and with the number k of FEC frames set as one cycle, interleaving is executed by n/m times for different information of each FEC frame.
 7. An FEC frame structuring method according to claim 1, wherein the first and second error correction codes are Reed-Solomon codes respectively represented by RS (q, r) and RS (p, q) (p, q and r are natural numbers, and p>q>r, p representing a code length of the second error correction code, q an information length of the second error correction code, and a code length of the first error correction code, and r an information length of the first error correction code).
 8. An FEC frame structuring method according to claim 2, wherein the first and second error correction codes are Reed-Solomon codes respectively represented by RS (q, r) and RS (p, q) (p, q and r are natural numbers, and p>q>r, p representing a code length of the second error correction code, q an information length of the second error correction code, and a code length of the first error correction code, and r an information length of the first error correction code).
 9. An FEC multiplexer comprising: first demultiplexing means for demultiplexing transmission information into parallel information; first speed conversion means for increasing a transmission speed by adding a redundant information region to the parallel information obtained by the demultiplexing of the first demultiplexing means; overhead insertion means for inserting overhead information into the redundant information region added by the first speed conversion means; first interleaving means for changing an order of the parallel information having the overhead information inserted thereinto by the overhead insertion means; first error correction coding means for generating a first error correction code for the overhead information of the parallel information, the order thereof having been changed by the first interleaving means, and transmission information, and then storing the generated first error correction code in the redundant information region; first deinterleaving means for rechanging the order, changed by the first interleaving means, of the parallel information having the first error correction code stored in the redundant information region by the first error correction coding means, to an original order; second error correction coding means for generating a second error correction code for the overhead information of the parallel information order-rechanged by the first deinterleaving means, the transmission information, and the first error correction code, and storing the generated second error correction code in the redundant information region; first multiplexing means for multiplexing the parallel information having the second error correction code stored in the redundant information region by the second error correction coding means, and generating an FEC frame; second demultiplexing means for demultiplexing the FEC frame, generated by the first multiplexing means and transmitted through an optical transmission path, into parallel information; frame alignment means for detecting a head position of the parallel information according to the overhead information stored in the redundant information region of the parallel information obtained by the demultiplexing of the second demultiplexing means, second error correction decoding means for correcting an error of the parallel information by decoding the second error correction code stored in the redundant information region of the parallel information synchronized by the frame alignment means; second interleaving means for changing, similarly to that by the first interleaving means, an order of the parallel information error-corrected by the second error correction decoding means; first error correction decoding means for correcting a residual error of the parallel information by decoding the first error correction code stored in the redundant information region of the parallel information order-changed by the second interleaving means; second deinterleaving means for rechanging the order, changed by the second interleaving means, of the parallel information error-corrected by the first error correction decoding means, to an original order; overhead separation means for separating overhead information stored in the redundant information region of the parallel information order-rechanged by the second deinterleaving means; second speed conversion means for reducing a transmission speed by erasing the redundant information region of the parallel information having the overhead information separated by the overhead separation means; and second multiplexing means for multiplexing the parallel information having the redundant information region erased by the second speed conversion means, and outputting transmission information.
 10. An FEC multiplexer according to claim 9, wherein error correction means composed of the second error correction decoding means, the second interleaving means, the first error correction decoding means, and the second deinterleaving means is connected in a multistage and cascaded manner. 